Molecular sieve zeolites are crystalline materials which are capable of adsorbing large quantities of molecules which are of sufficiently small size to pass into the uniformly-sized openings of the zeolite crystal structure. The zeolites are widely used as adsorbents for separating small molecules from large molecules, and as size-selective catalysts and catalyst supports. One purpose for which the zeolites show promise is the removal of sulfur dioxide from acidic gas streams, such as the effluent from smelters, acid-producing facilities, and fossil-fuel burning, steam-generation units. To qualify for this use, a zeolite must be acid stable and available in a particulate form which is adapted for economic gas contacting. Zeolites which are acid stable include mordenite and ferrierite. In the past, acid stable particles of zeolite have been generally restricted to pellets in the 1/8 inch diameter size range and have been subject to attrition-resistance weaknesses which limit them to use in fixed-bed adsorption systems. These pellets are formed of zeolite crystals which are normally in the five micrometer size range.
The use of a fluidized bed system for contacting solid adsorbents with gas streams has many well-known advantages over fixed bed systems. In a fluidized system, a mass of fluidizable particles (particles in the size range of 20 to 200 micrometers) is suspended in an upwardly-moving stream of the gas to be contacted. The result is extremely effective contact adsorption and good heat transfer throughout the mass of particles, due to the vigorous agitation of the "boiling" system. Fluidized beds for use in acidic gas streams must satisfy three basic requirements. First, the particle must be of the proper size to allow them to be fluidizable. Second, the particles must be attrition-resistant, since the vigorous agitation of the particles provide substantial shock and abrasive contact between particles. Third, the particle itself must be acid stable in order to avoid physical destruction in the acidic stream of gas.
A fourth requirement results from the fact that adsorption operations often require ion-exchange of the zeolite. If the particle is not resistant to attack by the ion-exchange solutions, the ion-exchange must be carried out before the particle is formed.
Previous attempts to provide a zeolite particle which satisfies the above requirements have usually involved forming suitably-sized particles from smaller crystals of zeolite which have been aggregated with clay binders. In addition to having low attrition-resistance, the binders were not resistant to acid and other chemical attack, so that the particles often broke down in use. Attempts at forming fluidizable particles of pure zeolite have generally been restricted to the less acid stable zeolite varieties. Thus, either the particles or the zeolite crystal itself will not withstand the environment of many potential uses. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide a zeolite particle in the fluidizable size range, which particle is both attrition-resistant and resistant to acid and other chemical attack.
Another object of this invention is the provision of a zeolite particle having both high adsorption capacity and high adsorption rates.
It is another object of the present invention to provide a zeolite particle which has practical value as an adsorbent, catalyst support, and ion-exchange medium.
A further object of the present invention is the provision of a zeolite particle which is manufactured in a simple and economic system.
With the foregoing and other objects in view, as will appear as the description proceeds, the invention resides in the combination and arrangement of process steps and the details of the composition hereinafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made without the scope of what is claimed without departing from the spirit of the invention.